Abstract: Objective: To investigate the effects of simulated weightlessness on lung function in healthy rats. Methods: Experiments were conducted on 40 Sprague-Dawley male rats,divided into 4 groups:suspended for 7 d,control of 7 d and suspended for 21 d,control of 21 d.Tail-suspension（-30° ） was used as weightlessness simulation.Lung function were studied with animal plethysmography when suspension ended.Test results were compared statistically. Results: Forced vital capacity（FVC）, FEV_0.3%, Peak expiratory flow（PEF） and Maximal voluntary ventilation（MVV） tended to decrease,especially FEV_0.3% in 7 d and MVV in 21 d decreased significantly （i>P＜0.05）.Inspiratory resistance（Ri） and Expiratory resistance（Re） tended to increase and Ri in 7 d increased significantly（i>P＜0.05）.Compliance of lung（Cl） had not a prominent change. Conclusion: Simulated weightlessness in tail-suspended rats has certain effects on lung function.